Design of a miniature solid state feed-thru nuller

A CW (continuous wave) radar receiver utilizing a solid-state feed-through nulling (FTN) system and providing 50 dB of signal cancellation over a 3% frequency band centered in X-band, has been developed. The transmits sample is amplitude and phase adjusted with a feedback system to be equal in magnitude and opposite in phase to the feed-through. This results in up to 60 dB of reduction in the feed-through signal level before it reaches the receiver´s active front-end components. FTN cancels AM and FM noise sidebands on the feed-through by the same amount that the carrier is canceled, allowing CW radars to operate with reasonable transmitter output purity. The noise performance of the feed-through nuller proved to be the most difficult problem to solve, and the performance of the present design slightly degrades the noise figure of the radar receiver. Work is currently underway to improve this noise performance.<>